While you haven't explicitly violated any single section of the NEC (provided you meet all the other requirements for the components and their installation), connecting bus duct sections with cable in the manner you described is generally highly discouraged and potentially problematic for several reasons

1. Mechanical Support and Strain:
Bus Duct Design

Bus duct is designed to be a mechanically rigid, self-supporting system. Cables, even with appropriate lugs and terminations, introduce a flexible link. This flexibility can place undue stress on the bus duct terminations (and the cable terminations) due to vibration, thermal expansion/contraction, or physical impacts. The bus duct system is designed for specific mechanical stresses that are not taken into account with the cable connection.

Cable Support

The cables themselves will require significant support to prevent sag and strain on the terminations. This support needs to be independent of the bus duct system.

Seismic Considerations

In areas prone to earthquakes, this design is even more problematic. The differential movement between the bus duct sections could damage the cables and terminations.

2. Termination Issues



Dissimilar Materials

Bus duct conductors are typically copper or aluminum bars. Cables are typically copper conductors with insulation. Connecting these dissimilar materials directly can lead to corrosion and increased resistance at the connection point, especially with temperature fluctuations. Even with appropriate lugs designed for connecting dissimilar metals, the long-term reliability can be a concern.

Lug Compatibility

Ensuring the lugs are properly sized and torqued for both the cable and the bus duct termination points is crucial. This requires careful attention to the manufacturer's specifications.

Enclosure Space

The tap boxes will need to be large enough to accommodate the cable terminations, bending radii, and any required splicing.

3. Ampacity and Heat Dissipation



Derating

While the individual components may have the same ampacity, the cable's ampacity might need to be derated based on the number of conductors in the same raceway or enclosure (tap box) and the ambient temperature. The bus duct ampacity calculation already takes into account the enclosure and heat dissipation of the bus duct.

Heat Build-up

Cables enclosed within the tap boxes can contribute to heat build-up, potentially affecting the bus duct's ampacity and the cable's insulation life.

Impedance

The cable connection will introduce a different impedance into the system. This could affect fault current distribution and coordination of overcurrent protection devices.

4. Code Compliance Concerns



NEC 368

While you may not find a direct prohibition, the intent of NEC Article 368 (Busways) is to provide a continuous, rigid, and easily accessible power distribution system. Introducing a flexible cable link deviates from this intent.

NEC 110.3(B)

This section requires equipment to be installed and used in accordance with its listing and labeling. The bus duct listing likely does not contemplate or approve the use of cable sections in this manner. The bus duct manufacturers need to provide installation instructions that include cable connections.

5. Practical Considerations



Cost

By the time you factor in the cost of suitable tap boxes, heavy-duty cable, appropriate lugs, cable supports, and skilled labor, this approach might not be cost-effective compared to using another bus duct section or a different wiring method altogether.

Future Modifications

This design makes it more difficult to modify or expand the bus duct system in the future.

Inspection

An electrical inspector may raise concerns about the unconventional nature of this installation and require detailed calculations and justifications to ensure safety and code compliance.

Why You Might Be Considering This

Common reasons for wanting to do this include:

Obstruction

Needing to navigate around a physical obstruction (like a pipe or HVAC duct) that would otherwise require a custom bus duct fitting.

Misalignment

Correcting a minor misalignment between bus duct sections.

Cost

Attempting to save money compared to purchasing a specialized bus duct fitting.

Better Alternatives

Instead of using cables, consider these alternatives:

Custom Bus Duct Fitting

The best solution is usually to order a custom-made bus duct fitting (e.g., an elbow, offset section, or flexible bus duct section) from the bus duct manufacturer. This will maintain the integrity of the bus duct system and ensure proper mechanical support and electrical continuity.

Rigid Conduit

If an obstruction absolutely cannot be avoided, transition from the bus duct to a suitable section of rigid metal conduit (RMC) or intermediate metal conduit (IMC) using appropriate fittings. This provides a more robust and mechanically sound alternative to cables. Be sure to properly size the conduit, conductors, and overcurrent protection.

Relocate Equipment

If the problem is misalignment, consider relocating the equipment connected to the bus duct to eliminate the need for the cable run.

Conclusion

While you might be able to technically argue that this design is not explicitly prohibited by the NEC, it is

not a good engineering practice and is likely to be problematic in the long run. The potential for mechanical stress, termination issues, heat build-up, and code compliance concerns makes it a riskier and less reliable solution than the alternatives. Consult with a qualified electrical engineer to explore the best solution for your specific situation. It is also best to consult with the bus duct manufacturer and your local electrical inspector. Flag for review